In January 1998 the European Telecommunications Standards Institute (ETSI) selected the basic technology for the Universal Mobile Telecommunications System (UMTS) (see ETSI, “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission”, June 1998). The main radio interface proposed was the WCDMA Wideband Code Division Multiple Access protocol, the properties of which allow satisfying the third-generation (3G) mobile telephony requirements in full. Due to the high data transmission rate and the increasingly demanding Quality of Service (QoS) requirements, it became necessary to develop new planning strategies. Among these, the one that is possibly the object of a larger number of studies is the power control system, specifically the procedure used to implement the outer loop of said system.
A general overview is given below of said power control system, as the functionality of the outer loop, for which this invention proposes a method, results from other system components.
Power control systems are needed in WCMDA-based cellular networks because this technology is limited by interference, as all users share the same frequency spectrum and their codes are not fully orthogonal (see Holma & Toskala: “WCDMA for UMTS, Radio Access for Third Generation Mobile Communications”, John Wiley & Sons).
The ultimate purpose of the power control system in WCDMA is to attain the required Quality of Service for a specific downlink from the base station to the mobile unit or terminal unit, or an uplink from the mobile unit to the base station, with a minimal transmitted power (the latter aspect is what the invention is centred upon).
The main objectives of the power control system in WCDMA networks are:
Cancelling the near-far effect: if all mobile stations transmit at the same power, without considering the distance or fading to the base station, the mobile units nearest the station will create significant interference for more distant terminals.
Protecting against deep fading.
Minimising interference in the network, with the resulting improvement in capacity.
Greater duration of mobile station batteries.
A WCMDA power control system is jointly implemented by three differentiated procedures:
Open loop: during the initial random access process of a connection, the base/mobile station estimates the power loss in the uplink/downlink and adjusts the transmission power accordingly.
Closed or internal loop, also known as fast power control (1500 Hz). Comprises the following three steps:    1) The corresponding receiver (base station or mobile unit) compares the desired signal—received interference ratio (SIRrec) with the target desired signal—interference ratio (SIRtarget), which depends on the quality of service required for this specific link and is determined by the open loop procedure explained further below.    2) The same receiver terminal sends power control bits indicating the transmission power must be increased (if SIRrec<SIRtarget) or reduced (if SIRrec>SIRtarget) by a certain amount (normally 1 dB).    3) The transmitter unit (base station or mobile unit) increases or reduces its power by the previously determined amount.
Outer Loop Power Control (OLPC). This is a much slower method than closed loop (10-100 Hz) and determines the target desired signal-interference ratio (SIRtarget) that will maintain a predetermined quality goal. One criterion or measure of link quality is the Frame Error Rate (FER) or, equivalently, the Block Error Rate (BLER), which depends on the desired signal to interference ratio (SIR). As the inner loop allows maintaining the desired received signal-interference ratio (SIRrec) near the target ratio (SIRtarget), the block error rate (BLER) is ultimately determined by this target value. In this way, to attain a certain quality of service in a given fade environment, SIRtarget must be adjusted to the appropriate value for that environment.
At the start of each transmission, whether voice or data, during a communication or call, the aforementioned outer loop power control (OLPC) usually sets SIRtarget to a very high value, in order to ensure a secure communication (see the patent by Tao Chen and Stein Lundby, US2005/0176456 of 17 Mar. 2005 entitled “Systems and methods for performing Outer Loop Power control in wireless communication systems”), then waiting for the aforementioned OLPC algorithm to reduce the value of SIRtarget to the suitable value for fulfilling the BLER corresponding to the service demanded. This process of reducing the SIRtarget at the start of each transmission, at the end of which the SIRtarget value approaches an ideal value, or the required signal-interference ratio (SIRreq), is known as the initial convergence.
The most widely used design for outer loop power control (OLPC) is based on target block error rate (BLERtarget) and known as BLER-Based OLPC. It measures this rate and changes SIRtarget accordingly, depending on whether BLERtarget is above or below the desired threshold (see Sampath A, Kumar P S & Holtzman J M (1997), “On setting reverse link target SIR in a CDMA system”, Proceedings of the IEEE Vehicular Technology Conference, Phoenix, Ariz., p 929-933). The problem is that due to the characteristics of the OLPC algorithm commonly used (see Holma H., Toskala A., “WCDMA for UMTS”, Wiley, 2002) the process of diminishing SIRtarget is very slow. This slow convergence is due to the fact that the down step size used by the algorithm, measured in dBs is on the order of the target frame error rate (FERtarget) (typical values are 10−2 for voice service and 10−3 for video calls), i.e. very small. Therefore, several tens of seconds are needed for each reduction by one dB.
Therefore, the aforementioned process of initial convergence in WCDMA is very slow, meaning that the SIRtarget fixed by the BLER-Based OLPC algorithm is greater than needed for a long time, with the resulting increase in interference and therefore loss of system capacity.
The problem is more marked in cases of discontinuous transmission services, in which the communication channel is used intermittently so that the initial convergence process takes place many times, as the transmission is constantly being ended and started, so that there are periods of silence between one transmission and the next within one voice or data call. Because the aforementioned process is very slow, as described above, it can be inferred that the known outer loop power control method, BLER-based OLPC, is inadequate for this type of service (see again the patent by Tao Chen & Stein Lundby number US2005/0176456 of 17 Mar. 2005 with title: “Systems and methods for performing Outer Loop Power control in wireless communication systems”).
Much research has been conducted on solving the slow convergence of the usual power control method which, as explained, takes place in BLER-Based OLPC. In this sense, it is worth mentioning Spanish Patent Application ES 2249192, filed by the applicant of the present invention. In the former invention, as described in ES 2249192, an outer loop power control method is proposed known as “Outage-Based OLPC”, in which the SIRtarget is calculated as the sum of two components, SIRtarget=SIRoutage-tgt+SIRBLER-tgt, by a dynamic adjustment function that establishes a correspondence between the quality criterion based on the target block error rate (BLERtarget) and an additional quality criterion based on outage probabilities. The first component (SIRoutage-tgt) adapts to the changing propagation conditions of the channel, so that it must be able to vary quickly. However, the second component (SIRBLER-tgt) is in charge of guaranteeing the target block error rate (BLERtarget) and therefore must maintain the characteristic step process of the known method, BLER-based OLPC (see again Sampath A, Kumar P S & Holtzman J M (1997), “On setting reverse link target SIR in a CDMA system”, Proc. IEEE Vehicular Technology Conference, Phoenix, Ariz., p 929-933).
Precisely for this reason, the last component (SIRBLER-tgt) of the SIRtarget has slow response characteristics, due to the BLER-based OLPC, as it includes the effects on the BLER that are not related to propagation conditions but instead to other parameters that depend more on the mobile terminal than the communications channel.